Load Control Device Having a Controllable Filter Circuit

ABSTRACT

A load control device may be configured to control an electrical load, such as a lighting load. The load control device may include a first terminal adapted to be coupled to an alternating-current (AC) power source, and a second terminal adapted to be coupled to the electrical load. The load control device may include a bidirectional semiconductor switch, a filter circuit, and a control circuit. The bidirectional semiconductor switch may be coupled in series between the first terminal and the second terminal, and be configured to provide a phase-control voltage to the electrical load. The filter circuit may be coupled between the first terminal and the second terminal. The control circuit may be configured to render the bidirectional semiconductor switch conductive and non-conductive to control an amount of power delivered to the electrical load, and be configured to adjust the impedance and/or filtering characteristics of the filter circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/689,910, filed Jun. 26, 2018, the entire disclosureof which is hereby incorporated by reference.

BACKGROUND

Prior art two-wire load control devices, such as dimmer switches, arecoupled in series electrical connection between an alternating-current(AC) power source and a lighting load for controlling the amount ofpower delivered from the AC power source to the lighting load. Atwo-wire wall-mounted dimmer switch is adapted to be mounted to astandard electrical wallbox and comprises two load terminals: a hotterminal adapted to be coupled to the hot side of the AC power sourceand a dimmed hot terminal adapted to be coupled to the lighting load. Inother words, the two-wire dimmer switch does not require a connection tothe neutral side of the AC power source (i.e., the load control deviceis a “two-wire” device). Prior art “three-way” dimmer switches may beused in three-way lighting systems and comprise at least three loadterminals, but do not require a connection to the neutral side of the ACpower source.

The dimmer switch typically comprises a bidirectional semiconductorswitch, e.g., a thyristor (such as a triac) or two field-effecttransistors (FETs) in anti-series connection. The bidirectionalsemiconductor switch is coupled in series between the AC power sourceand the load and is controlled to be conductive and non-conductive forportions of a half cycle of the AC power source to thus control theamount of power delivered to the electrical load. Generally, dimmerswitches use either a forward phase-control dimming technique or areverse phase-control dimming technique in order to control when thebidirectional semiconductor switch is rendered conductive andnon-conductive to thus control the power delivered to the load. Thedimmer switch may comprise a toggle actuator for turning the lightingload on and off and an intensity adjustment actuator for adjusting theintensity of the lighting load. Examples of prior art dimmer switchesare described in greater detail is commonly-assigned U.S. Pat. No.5,248,919, issued Sep. 29, 1993, entitled LIGHTING CONTROL DEVICE; U.S.Pat. No. 6,969,959, issued Nov. 29, 2005, entitled ELECTRONIC CONTROLSYSTEMS AND METHODS; and U.S. Pat. No. 7,687,940, issued Mar. 30, 2010,entitled DIMMER SWITCH FOR USE WITH LIGHTING CIRCUITS HAVING THREE-WAYSWITCHES, the entire disclosures of which are hereby incorporated byreference.

With forward phase-control dimming, the bidirectional semiconductorswitch is rendered conductive at some point within each AC line voltagehalf cycle and remains conductive until approximately the next voltagezero-crossing of the AC line voltage, such that the bidirectionalsemiconductor switch is conductive for a conduction time each halfcycle. A zero-crossing is defined as the time at which the AC linevoltage transitions from positive to negative polarity, or from negativeto positive polarity, at the beginning of each half cycle. Forwardphase-control dimming is often used to control energy delivered to aresistive or inductive load, which may include, for example, anincandescent lamp or a magnetic low-voltage transformer. Thebidirectional semiconductor switch of a forward phase-control dimmerswitch is typically implemented as a thyristor, such as a triac or twosilicon-controlled rectifiers (SCRs) coupled in anti-parallelconnection, since a thyristor becomes non-conductive when the magnitudeof the current conducted through the thyristor decreases toapproximately zero amps.

When using reverse phase-control dimming, the bidirectionalsemiconductor switch is rendered conductive at the zero-crossing of theAC line voltage and rendered non-conductive at some point within eachhalf cycle of the AC line voltage, such that the bidirectionalsemiconductor switch is conductive for a conduction time each halfcycle. Reverse phase-control dimming is often used to control energy toa capacitive load, which may include, for example, an electroniclow-voltage transformer. Since the bidirectional semiconductor switchmust be rendered conductive at the beginning of the half cycle, and mustbe able to be rendered non-conductive within the half cycle, reversephase-control dimming requires that the dimmer switch have two FETs inanti-serial connection, or the like. A FET may be rendered conductiveand to remain conductive independent of the magnitude of the currentconducted through the FET. In other words, a FET is not limited by arated latching or holding current as is a thyristor. However, prior artreverse phase-control dimmer switches have either required neutralconnections and/or advanced control circuits (such as microprocessors)for controlling the operation of the FETs. In order to power amicroprocessor, the dimmer switch must also comprise a power supply,which is typically coupled in parallel with the FETs. These advancedcontrol circuits and power supplies add to the cost of prior artFET-based reverse phase-control dimmer switches, for example, ascompared to analog forward phase-control dimmer switches.

Nevertheless, it is desirable to be able to control the amount of powerto electrical loads having power rating lower than those able to becontrolled by the prior art forward and reverse phase-control dimmerswitches. In order to save energy, high-efficiency lighting loads, suchas, for example, compact fluorescent lamps (CFLs) and light-emittingdiode (LED) light sources, are being used in place of or as replacementsfor conventional incandescent or halogen lamps. High-efficiency lightsources typically consume less power and provide longer operationallives as compared to incandescent and halogen lamps. In order toilluminate properly, a load regulation device (e.g., such as anelectronic dimming ballast or an LED driver) must be coupled between theAC power source and the respective high-efficiency light source (e.g.,the compact fluorescent lamp or the LED light source) for regulating thepower supplied to the high-efficiency light source.

A dimmer switch controlling a high-efficiency light source may becoupled in series between the AC power source and the load controldevice for the high-efficiency light source. Some high-efficiencylighting loads are integrally housed with the load regulation devices ina single enclosure. Such an enclosure may have a screw-in base thatallows for mechanical attachment to standard Edison sockets, and provideelectrical connections to the neutral side of the AC power source andeither the hot side of the AC power source or the dimmed-hot terminal ofthe dimmer switch (e.g., for receipt of the phase-control voltage). Theload regulation circuit may be configured to control the intensity ofthe high-efficiency light source to the desired intensity in response tothe conduction time of the bidirectional semiconductor switch of thedimmer switch.

A dimmer switch for controlling a high-efficiency light source may beconfigured for constant gate drive, where a control circuit of thedimmer switch provides constant gate drive to the bidirectionalsemiconductor switch so the bidirectional semiconductor switch remainsconductive independent of the magnitude of the load current. Forexample, the dimmer switch may use a FET to keep the triac conductive byensuring that the gate current is above the holding current of thetriac. Examples of such a dimmer switch are described in greater detailin commonly-assigned U.S. Pat. No. 8,664,881, issued Mar. 4, 2014,entitled TWO-WIRE DIMMER SWITCH FOR LOW-POWER LOADS, the entiredisclosure of which is hereby incorporated by reference.

Further, a dimmer switch used for controlling high-efficiency lightsources have a smaller radio-frequency interference (RFI) capacitor thandimmer switches used for controlling traditional light sources (e.g.,incandescent loads). The reduction in size of the RFI capacitor was donefor a couple reasons. For example, the larger RFI capacitors wouldsometimes create a bias current that would cause the load regulationdevice to illuminate the controlled high-efficiency light source to alevel that is perceptible by the human eye when the light source shouldbe off. Further, and for example, the larger RFI capacitors tended tophase-shift the output current of the dimmer switch, and thisphase-shift would interfere with start-up of the high-efficiency lightsource. As such, dimmer switches used for controlling high-efficiencylight sources tend to have smaller RFI capacitors.

Additionally, the load regulation devices for the high-efficiency lightsources may have high input impedances or input impedances that vary inmagnitude throughout a half cycle. Therefore, when a prior-art forwardphase-control dimmer switch is coupled between the AC power source andthe load regulation device for the high-efficiency light source, theload control device may not be able to conduct enough current to exceedthe rated latching and/or holding currents of the thyristor. Inaddition, when a prior-art reverse phase-control dimmer switch iscoupled between the AC power source and the load regulation device, themagnitude of the charging current of the power supply may be greatenough to cause the load regulation device to illuminate the controlledhigh-efficiency light source to a level that is perceptible by the humaneye when the light source should be off.

The impedance characteristics of the load regulation device maynegatively affect the magnitude of the phase-control voltage received bythe load regulation device, such that the conduction time of thereceived phase-control voltage is different from the actual conductiontime of the bidirectional semiconductor switch of the dimmer switch(e.g., if the load regulation device has a capacitive impedance).Therefore, the load regulation device may control the intensity of thehigh-efficiency light source to an intensity that is different than thedesired intensity as directed by the dimmer switch. In addition, thecharging current of the power supply of the dimmer switch may build upcharge at the input of a load regulation device having a capacitiveinput impedance, thus negatively affecting the low-end intensity thatmay be achieved.

SUMMARY

As described herein, a load control device (e.g., a dimmer switch) forcontrolling an electrical load (e.g., a lighting load) may comprise acontrollable filter circuit that may be controlled to adjust filteringcharacteristics of the controllable filter circuit based on one or morefactors. The load control device may include a first terminal adapted tobe coupled to an alternating-current (AC) power source, and a secondterminal adapted to be coupled to the electrical load. The load controldevice may also include a bidirectional semiconductor switch (e.g., athyristor) coupled in series between the first terminal and the secondterminal. The bidirectional semiconductor switch may be configured to becontrolled to a conductive state and a non-conductive state. Thecontrollable filter circuit may be coupled between the first terminaland the second terminal. Further, the load control device may include acontrol circuit configured to render the bidirectional semiconductorswitch conductive and non-conductive to control an amount of powerdelivered to the electrical load. The control circuit may be furtherconfigured to adjust an impedance (e.g., a capacitance and/or aresistance) of the controllable filter circuit. In some examples, thecontrollable filter circuit may be used for radio-frequency interference(RFI) filtering.

The controllable filter circuit may include one or more switches thatmay be controlled by the control circuit to adjust the impedance, and inturn the filtering characteristics, of the filter circuit. Thecontrollable filter circuit may also be coupled between thebidirectional semiconductor switch and the second terminal of the loadcontrol device. The control circuit may be configured to adjust theimpedance of the controllable filter circuit based on a state (e.g., apower state) of the bidirectional semiconductor switch, during a turn-onperiod after the load control device receives an input to provided powerto the electrical load, and/or based on the amount of power delivered tothe electrical load. The load control device may also comprise ameasurement circuit configured to generate a feedback signal indicatinga magnitude of a voltage developed across the load control device. Thecontrol circuit may be configured to measure a slope of the feedbacksignal when the bidirectional semiconductor switch is transitioning fromthe non-conductive state to the conductive state, and adjust theimpedance of the controllable filter circuit in response to the slope ofthe feedback signal.

In addition, the filter circuit may include an inductor, one or morecapacitors, one or more resistors, and/or one or more controllableswitches. The inductor may be coupled in series between thebidirectional semiconductor switch and the second terminal. A capacitorand a switch of the filter circuit may be coupled in series between thefirst terminal and the second terminal, for example, such that thecontrol circuit may be configured to render the switch conductive andnon-conductive to take the capacitor in and out of series connectionbetween the first terminal and the second terminal to adjust theimpedance of the filter circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of an example lighting controlsystem including a load control device (e.g., a “two-wire” dimmerswitch) for controlling the intensity of a high-efficiency lighting load(e.g., an LED light source).

FIG. 2 is a simplified block diagram of an example dimmer switch.

FIGS. 3 and 4 are simplified schematic diagrams of example dimmerswitches.

FIGS. 5-8 are flowcharts of example filter circuit control proceduresthat may be performed by a load control device, such as a dimmer switch.

DETAILED DESCRIPTION

FIG. 1 is a simplified block diagram of an example load control system10 (e.g., a lighting control system) including a load control device,e.g., “two-wire” dimmer switch 100, for controlling the amount of powerdelivered to an electrical load, e.g., a lighting load 101. The lightingload 101 may comprise any suitable dimmable lighting load, such as, forexample, an incandescent lamp, a halogen lamp, an electronic low-voltagelighting load, a magnetic low-voltage lighting load, or other type oflighting load. In addition, as shown in FIG. 1, the lighting load 101may comprise, for example, a high-efficiency lighting load including aninternal load regulation device, e.g., a light-emitting diode (LED)driver 102, and a high-efficiency light source, e.g., an LED lightsource 104 (or “light engine”). The dimmer switch 100 may have a hotterminal H coupled to an alternating-current (AC) power source 105 forreceiving an AC mains line voltage V_(AC), and a dimmed-hot terminal DHcoupled to the LED driver 102. The dimmer switch 100 may not require adirect connection to the neutral side N of the AC power source 105. Thedimmer switch 100 may generate a phase-control voltage V_(PC) (e.g., adimmed-hot voltage) at the dimmed-hot terminal DH and conduct a loadcurrent I_(LOAD) through the lighting load 101. The dimmer switch 100may either use forward phase-control dimming or reverse phase-controldimming techniques to generate the phase-control voltage V_(PC).

As defined herein, a “two-wire” dimmer switch or load control devicedoes not require a require a direct connection to the neutral side N ofthe AC power source 105. In other words, all currents conducted by thetwo-wire dimmer switch will also be conducted through the load. Atwo-wire dimmer switch may have only two terminals (e.g., the hotterminal H and the dimmed hot terminal DH as shown in FIG. 1).Alternatively, a two-wire dimmer switch (as defined herein) may comprisea three-way dimmer switch that may be used in a three-way lightingsystem and may have at least three load terminals, but may not require aneutral connection. In addition, a two-wire dimmer switch may comprisean additional connection that may provide for communication with aremote control device (for remotely controlling the dimmer switch), butmay not require the dimmer switch to be directly connected to neutral.

The LED driver 102 and the LED light source 104 may be both includedtogether in a single enclosure, for example, having a screw-in baseadapted to be coupled to a standard Edison socket. When the LED driver102 is included with the LED light source 104 in the single enclosure,the LED driver may only have two electrical connections: to the dimmerswitch 100 for receiving the phase-control voltage V_(PC) and to theneutral side N of the AC power source 105. The LED driver 102 maycomprise a rectifier bridge circuit 106 that may receive thephase-control voltage V_(PC) and generate a bus voltage V_(BUS) across abus capacitor C_(BUS). The LED driver 102 may further comprise a loadcontrol circuit 107 that may receive the bus voltage V_(BUS) and controlthe intensity of the LED light source 104 in response to thephase-control signal V_(PC). Specifically, the load control circuit 107of the LED driver 102 may be configured to turn the LED light source 104on and off and to adjust the intensity of the LED light source to atarget intensity L_(TRGT) (e.g., a desired intensity) in response to thephase-control signal V_(PC). The target intensity L_(TRGT) may rangebetween a low-end intensity L_(LE) and a high-end intensity L_(HE). TheLED driver 102 may also comprise a filter network 108 for preventingnoise generated by the load control circuit 107 from being conducted onthe AC mains wiring. Since the LED driver 102 comprises the buscapacitor C_(BUS) and the filter network 108, the LED driver may have acapacitive input impedance. An example of the LED driver 102 isdescribed in greater detail in U.S. Pat. No. 8,492,987, issued Jul. 23,2013, entitled LOAD CONTROL DEVICE FOR A LIGHT-EMITTING DIODE LIGHTSOURCE, the entire disclosure of which is hereby incorporated byreference.

In addition, the LED driver 102 may comprise an artificial load circuit109 for conducting current (in addition to the load current I_(LOAD))through the dimmer switch 100. Accordingly, if the dimmer switch 100includes a triac for generating the phase-control voltage V_(PC), theartificial load circuit 109 may conduct enough current to ensure thatthe magnitude of the total current conducted through the triac of thedimmer switch 100 exceeds the rated latching and holding currents of thetriac. In addition, the artificial load circuit 109 may conduct a timingcurrent if the dimmer switch 100 comprises a timing circuit and mayconduct a charging current if the dimmer switch comprises a powersupply, such that these currents need not be conducted through the loadcontrol circuit 107 and do not affect the intensity of the LED lightsource 104.

The artificial load circuit 109 may simply comprise a constant impedancecircuit (e.g., a resistor) or may comprise a current source circuit.Alternatively, the artificial load circuit 109 may be controllable, suchthat the artificial load circuit may be enabled and disabled to thusselectively conduct current through the dimmer switch 100. In addition,the artificial load circuit 109 may be controlled to conduct differentamounts of current depending upon the magnitude of the AC mains linevoltage V_(AC), the present time during a half cycle of the AC mainsline voltage, or the present operating mode of the LED driver 102.Examples of artificial load circuits are described in greater detail incommonly-assigned U.S. Pat. No. 8,169,154, issued May 1, 2012,entitledVARIABLE LOAD CIRCUITS FOR USE WITH LIGHTING CONTROL DEVICES, and U.S.Patent Application Publication No. 2011/0121744, published May 26, 2011,entitled CONTROLLABLE-LOAD CIRCUIT FOR USE WITH A LOAD CONTROL DEVICE,the entire disclosures of which are hereby incorporated by reference.

Alternatively, the high-efficiency light source could comprise a compactfluorescent lamp (CFL) and the load regulation device could comprise anelectronic dimming ballast. In addition, the dimmer switch 100 couldalternatively control the amount of power delivered to other types ofelectrical loads, for example, by directly controlling a lighting loador a motor load. An example of a screw-in light source having afluorescent lamp and an electronic dimming ballast is described ingreater detail in U.S. Pat. No. 8,803,436, issued Aug. 12, 2014,entitled DIMMABLE SCREW-IN COMPACT FLUORESCENT LAMP HAVING INTEGRALELECTRONIC BALLAST CIRCUIT, the entire disclosure of which is herebyincorporated by reference.

The dimmer switch 100 may comprise a user interface having a rockerswitch 116 and an intensity adjustment actuator 118 (e.g., a slider knobas shown in FIG. 1). The rocker switch 116 may allow for turning on andoff the LED light source 104, while the intensity adjustment actuator118 may allow for adjustment of the target intensity L_(TRGT) of the LEDlight source 104 from the low-end intensity L_(LE) to the high-endintensity L_(HE). Examples of user interfaces of dimmer switches aredescribed in greater detail in commonly-assigned U.S. Pat. No.8,049,427, issued Nov. 1, 2011, entitled LOAD CONTROL DEVICE HAVING AVISUAL INDICATION OF ENERGY SAVINGS AND USAGE INFORMATION, the entiredisclosure of which is hereby incorporated by reference.

FIG. 2 is a simplified block diagram of an example dimmer switch 200,which may be deployed as the dimmer switch 100 of FIG. 1. The dimmerswitch 200 may comprise a bidirectional semiconductor switch 210, suchas a thyristor (e.g., a triac and/or one or more silicon-controlledrectifiers (SCRs)), a field-effect transistor (FET) in a full-waverectifier bridge, two FETs in anti-series connection, one or moreinsulated-gate bipolar junction transistors (IGBTs), or other suitableswitching circuit. The bidirectional semiconductor switch 210 may becoupled between a hot terminal H and a dimmed hot terminal DH forgenerating a phase-control voltage V_(PC) and conducting a load currentI_(LOAD) through an electrical load (e.g., the lighting load 101 shownin FIG. 1) for controlling of the amount of power delivered to theelectrical load. The bidirectional semiconductor switch 210 may comprisea first main terminal 222 electrically coupled to the hot terminal H anda second main terminal electrically 224 coupled to the dimmed-hotterminal DH.

The dimmer switch 200 may comprise a mechanical air-gap switch 212electrically coupled to the hot terminal H and in series with thebidirectional semiconductor switch 210. When the air-gap switch 212 isopen, the electrical load may be turned off. When the air-gap switch 212is closed, the dimmer switch 200 may be configured to control thebidirectional semiconductor switch 210 to control the amount of powerdelivered to the electrical load. The air-gap switch 212 may bemechanically coupled to an actuator of a user interface of the dimmerswitch 200 (e.g., the rocker switch 116), such that the switch may beopened and closed in response to actuations of the actuator.

The dimmer switch 200 may comprise a control circuit 214 configured tocontrol the bidirectional semiconductor switch 210 using a phase-controltechnique (e.g., a forward or reverse phase-control technique) tocontrol the amount of power delivered to the electrical load (e.g., tocontrol the intensity of the lighting load 101). When using the forwardphase-control technique, the control circuit 214 may render thebidirectional semiconductor switch 210 conductive at a firing time(e.g., at a firing angle) each half cycle of the AC power source. Thecontrol circuit 214 may be configured to adjust the firing time from onehalf-cycle to the next to control a target intensity L_(TRGT) from aminimum intensity L_(MIN) (e.g., approximately 1%) to a maximumintensity L_(MAX) (e.g., approximately 100%).

The dimmer switch 200 may comprise one or more actuators 216 (e.g., therocker switch 116 and an intensity adjustment actuator 118 of the dimmerswitch 100 shown in FIG. 1) for receiving user inputs and/or one or morevisual indicators 218 for providing feedback to a user of the dimmerswitch. The control circuit 214 may be configured to control thebidirectional semiconductor switch 210 in response to actuations of oneor more of the actuators 216 (e.g., to turn the lighting load 101 on andoff and/or adjust the intensity of the lighting load 101). The controlcircuit 214 may be configured to illuminate or more of the visualindicators 218 to provide feedback to the user (e.g., feedbackindicating the status and/or intensity level of the lighting load 101).

The dimmer switch 200 may comprise a controllable radio-frequencyinterference (RFI) filter circuit 220. The controllable RFI filtercircuit 220 may be electrically coupled between the hot terminal H andthe dimmed hot terminal DH. For example, the controllable RFI filtercircuit 220 may comprise one or more filter components (e.g., one ormore filter capacitors) coupled between the hot terminal H and thedimmed hot terminal DH. In addition, the controllable RFI filter circuit220 may be coupled between the second main terminal 224 of thebidirectional semiconductor switch 210 and the dimmed hot terminal DH.For example, the controllable RFI filter circuit 220 may comprise one ormore filter components (e.g., one or more filter inductors or chokes) inseries with the bidirectional semiconductor switch 210 (e.g., in serieswith the second main terminal 224 of the bidirectional semiconductorswitch).

The control circuit 214 may be coupled to the controllable RFI filtercircuit 220 for controlling filtering characteristics of thecontrollable RFI filter circuit 220 (e.g., an impedance or impedancelevel of the controllable RFI filter circuit between the hot terminal Hand the dimmed hot terminal DH and/or an impedance or impedance level ofthe controllable RFI filter circuit in series with the bidirectionalsemiconductor switch). For example, the control circuit 214 may beconfigured to control the controllable RFI filter circuit 220 to connectand disconnect a filter capacitor coupled between the hot terminal H andthe dimmed hot terminal DH. In addition, the control circuit 214 may beconfigured to control the controllable RFI filter circuit 220 to adjustthe capacitance of a filter capacitor coupled between the hot terminal Hand the dimmed hot terminal DH.

The control circuit 214 may control the controllable RFI filter circuit220 to operate as an LC filter circuit (e.g., an inductor-capacitorfilter circuit) or as an RLC filter circuit (e.g., aresistor-capacitor-inductor filter circuit). The controllable RFI filtercircuit 220 may be configured as an LC filter circuit if thecontrollable RFI filter circuit 220 comprises an inductive circuit(e.g., one or more filter inductors) and a capacitive circuit (e.g., oneor more filter capacitors). For example, the control circuit 214 may beconfigured to control the controllable RFI filter circuit 220 to connectthe capacitive circuit between the hot terminal H and the dimmed hotterminal DH and/or to connect the inductive circuit between the dimmedhot terminal DH and the bidirectional semiconductor switch. Thecontrollable RFI filter circuit 220 may also include a resistive circuit(e.g., one or more filter resistors), and for example, the controlcircuit may be configured to control the controllable RFI filter circuit220 to controllably connect and disconnect the resistive circuit fromseries connection with the capacitive circuit. If the resistive circuitis coupled in series with the capacitive circuit between the hotterminal H and the dimmed hot terminal DH, and the inductive circuit iscoupled between the dimmed hot terminal DH and the bidirectionalsemiconductor switch, the controllable RFI filter circuit 220 may beconfigured as an RLC filter circuit.

The control circuit 214 may be configured to control the controllableRFI filter circuit 220 in response to the state (e.g., the power state)of the electrical load. For example, the control circuit 214 may beconfigured to connect and/or increase the capacitance of the capacitivecircuit between the hot terminal H and the dimmed hot terminal DH whenthe electrical load is on (e.g., when the electrical load is in a firstpower state), and disconnect and/or decrease the capacitance of thecapacitive circuit between the hot terminal H and the dimmed hotterminal DH when the electrical load is off (e.g., when the electricalload is in a second power state).

In addition, the control circuit 214 may be configured to control thecontrollable RFI filter circuit 220 to provide a different impedancebetween the hot terminal H and the dimmed hot terminal DH while thedimmer switch 200 is turning on the lighting load (e.g., during aturn-on sequence) than while the dimmer switch 200 is in a steady statecondition. For example, the control circuit 214 may be configured todisconnect and/or decrease the capacitance of the capacitive circuitbetween the hot terminal H and the dimmed hot terminal DH during aturn-on period (e.g., a predetermined amount of time) after the air-gapswitch 212 is closed to turn on the lighting load, and connect and/orincrease the capacitance of the capacitive circuit between the hotterminal H and the dimmed hot terminal DH after the turn-on period(e.g., at the end of the turn-on period).

The control circuit 214 may be configured to control the controllableRFI filter circuit 220 to provide a different impedance between the hotterminal H and the dimmed hot terminal DH during different portions ofthe dimming range of the dimmer switch 200. For example, the controlcircuit 214 may to connect and/or increase the capacitance of thecapacitive circuit between the hot terminal H and the dimmed hotterminal DH near the middle of the dimming range (e.g., when the targetintensity L_(TRGT) between 25% and 75%), and disconnect and/or decreasethe capacitance of the capacitive circuit between the hot terminal H andthe dimmed hot terminal DH during the other portions of the dimmingrange (e.g., when the target intensity L_(TRGT) less than 25% andgreater than 75%). That is, the control circuit may determine if theintensity level L_(TRGT) is between a low intensity threshold L_(LOW)(e.g., 25% intensity) and a high intensity threshold L_(HIGH) (e.g., 75%intensity), and increase the capacitance of the capacitive circuitbetween the hot terminal H and the dimmed hot terminal DH.

FIG. 3 is a simplified block diagram of another example dimmer switch300 (e.g., an analog dimmer switch), which may be deployed as the dimmerswitch 100 of FIG. 1 and/or the dimmer switch 200 of FIG. 2. The dimmerswitch 300 may comprise a bidirectional semiconductor switch, e.g., athyristor, such as, a triac 310, which may be coupled between a hotterminal H and a dimmed hot terminal DH. The triac 310 may comprise afirst main terminal electrically coupled to the hot terminal H and asecond main terminal electrically coupled to the dimmed-hot terminal DH.The triac 310 may comprise a gate terminal (e.g., a control input),which may receive control signals for rendering the triac conductive.Alternatively, the bidirectional semiconductor switch of the dimmerswitch 300 may comprise a field-effect transistor (FET) in a rectifierbridge, two FETs electrically coupled in anti-series connection, and/orone or more insulated gate bipolar junction transistors (IGBTs).

The dimmer switch 300 may comprise a mechanical air-gap switch 312electrically coupled to the hot terminal H and in series with the triac310, such that the electrical load may be turned off when the switch isopen. The air-gap switch 312 may be mechanically coupled to an actuatorof a user interface of the dimmer switch 300 (e.g., the rocker switch116), such that the switch may be opened and closed in response toactuations of the actuator.

When the air-gap switch 312 is closed, the triac 310 may be controlledto generate a phase-control voltage V_(PC) (e.g., a forwardphase-control voltage) and conduct a load current I_(LOAD) through anelectrical load (e.g., the lighting load 101 shown in FIG. 1) forcontrolling of the amount of power delivered to the electrical load. Thetriac 310 may become non-conductive when the magnitude of the loadcurrent I_(LOAD) conducted through the triac drops below a rated holdingcurrent of the triac. The phase-control voltage V_(PC) may have amagnitude approximately equal to zero volts at the beginning of eachhalf cycle during a non-conduction time T_(NC), and may have a magnitudeapproximately equal to the magnitude of the AC line voltage V_(AC) ofthe AC power source 105 during the rest of the half cycle, e.g., duringa conduction time T_(CON).

The dimmer switch 300 may comprise a control circuit 314 (e.g., ananalog control circuit) for controlling the triac 310. The controlcircuit 314 may be coupled in parallel with the triac 310 (e.g., coupledbetween the first and second main terminals of the triac 310). Thecontrol circuit 314 may comprise a timing circuit including apotentiometer R315 and a capacitor C316 that are coupled in seriesbetween the first and second main terminals of the triac 310. Thejunction of the potentiometer R315 and the capacitor C316 may be coupledto the gate of the triac 310 through a triggering circuit, such as adiac 318. The wiper of the potentiometer R315 may be coupled to thejunction of the potentiometer and the capacitor C316, such that thepotentiometer R315 provides a variable resistance between the first mainterminal of the triac 310 and the capacitor C316. The position of thewiper of the potentiometer R315 may be adjusted by an intensityadjustment actuator of the dimmer switch 300 (e.g., the intensityadjustment actuator 118 of the dimmer switch 100 shown in FIG. 1). Thecapacitor C316 begins to charge through the potentiometer R315 at thebeginning of each half-cycle. When the voltage across the capacitor C316exceeds a breakover voltage of the diac 318 (e.g., at the firing time),the diac may be configured to conduct a pulse of gate current throughthe gate of the triac 310, thus rendering the triac conductive. The rateat which the capacitor C316 charges, and thus the firing time of thetriac 310 may be adjusted by varying the resistance provided by thepotentiometer R315 between the first main terminal of the triac 310 andthe capacitor C316.

The dimmer switch 300 may further comprise a controllable RFI filtercircuit 320. The controllable RFI filter circuit 320 may comprise aninductor L322 (e.g., a filter inductor or choke) coupled in series withthe second main terminal of the triac 310. The controllable RFI filtercircuit 320 may comprise a first capacitor C324 (e.g., a first filtercapacitor) and a controllable switch 325 that are electrically coupledin series between the hot terminal H and the dimmed hot terminal DH. Thecontrol circuit 314 may be configured to render the controllable switch325 conductive and non-conductive to respectively connect and disconnectthe first capacitor C324 from the series connection between the hotterminal H and the dimmed hot terminal DH. The controllable switch 325may be a single transistor (e.g., a FET), an optocoupler, a relay, oranother type of controllable switching circuit. For example, if thecontrollable switch 325 comprises a single FET, the FET may be renderednon-conductive to prevent the first capacitor C324 from charging duringthe positive half-cycles. In the negative half-cycles, the firstcapacitor C324 may be configured to charge through the body diode of theFET, but may not discharge since the FET is non-conductive. As a result,the first capacitor C324 may charge to approximately the negative peakof the AC mains line voltage V_(AC) and a magnitude of a leakage currentconducted through the lighting load may be approximately zero amps.

The controllable RFI filter circuit 320 may further comprise a secondcapacitor C326 (e.g., a second filter capacitor) coupled between the hotterminal H and the dimmed hot terminal DH and in parallel with theseries combination of the first capacitor C324 and the controllableswitch 325. The control circuit 314 may be configured to render thecontrollable switch 325 non-conductive to control the capacitancebetween the hot terminal H and the dimmed hot terminal DH to a firstvalue (e.g., by only coupling the second capacitor C326 between the hotterminal H and the dimmed hot terminal DH). The control circuit 314 maybe configured to render the controllable switch 325 conductive tocontrol (e.g., increase) the capacitance between the hot terminal H andthe dimmed hot terminal DH to a second value (e.g., by coupling thefirst and second capacitors C324, C326 in parallel between the hotterminal H and the dimmed hot terminal DH). The first capacitor C324,the controllable switch 325, and the second capacitor C326 may form acapacitive circuit, e.g., a controllable capacitive circuit.

The control circuit 314 may be configured to control the controllableswitch 325 of the controllable RFI filter circuit 320 in response to thestate of the electrical load (e.g., the state of the triac 310). Forexample, the control circuit 314 may be configured to adjust thecapacitance provided between the hot terminal H and the dimmed hotterminal DH during a turn-on period after the air-gap switch 312 isclosed to turn on the electrical load. For example, the control circuit314 may be configured to render the controllable switch 325non-conductive to couple just the second capacitor C326 between the hotterminal H and the dimmed hot terminal DH during the turn-on periodafter the air-gap switch 312 is closed. The control circuit 314 may beconfigured to render the controllable switch 325 conductive to couplethe first and second capacitors C324, C326 in parallel between the hotterminal H and the dimmed hot terminal DH after the end of the turn-onperiod (e.g., when the dimmer switch 300 is in a steady-statecondition).

The control circuit 314 comprise a delay circuit 330 configured torender the controllable switch 325 conductive after the turn-on period.The delay circuit 330 between the first and second main terminals of thetriac 310 and may be responsive to the voltage generated across thetriac 310. When the triac 310 is rendered conductive, the voltage acrossthe triac 310 may drop to a small voltage (e.g., approximately one volt)at which time the delay circuit 330 may begin the turn-on period. At theend of the turn-on period, the delay circuit 330 may render thecontrollable switch 325 conductive to couple the first and secondcapacitors C324, C326 in parallel between the hot terminal H and thedimmed hot terminal DH.

The second capacitor C326 may be optional. For example, if the secondcapacitor C326 is not included in the controllable RFI filter circuit320, the control circuit 314 may be configured to render thecontrollable switch 325 non-conductive to provide no capacitance betweenthe hot terminal H and the dimmed hot terminal DH and conductive toprovide some capacitance between the hot terminal H and the dimmed hotterminal DH (e.g., the capacitance of the capacitor C324). The controlcircuit 314 may be configured to control the controllable switch 325 ofthe controllable RFI filter circuit 320 in response to the delay circuit330. For example, the control circuit 314 may be configured to adjustthe capacitance provided between the hot terminal H and the dimmed hotterminal DH during a turn-on period after the air-gap switch 312 isclosed to turn on the electrical load. For example, the control circuit314 may be configured to render the controllable switch 325non-conductive to disconnect the first capacitor C324 during the turn-onperiod after the air-gap switch 312 is closed. The control circuit 314may be configured to render the controllable switch 325 conductive tocouple the first capacitor C324 between the hot terminal H and thedimmed hot terminal DH after the end of the turn-on period.

The control circuit 314 may be configured to control the controllableRFI filter circuit 320 to provide a different impedance between the hotterminal H and the dimmed hot terminal DH during different portions ofthe dimming range of the dimmer switch 300. For example, the controlcircuit may include additional circuitry (not shown) configured todetermine if the intensity level L_(TRGT) is between a low intensitythreshold L_(LOW) (e.g., 25% intensity) and a high intensity thresholdL_(HIGH) (e.g., 75% intensity). The control circuit 314 may close thecontrollable switch 325 to connect the capacitor C324 between the hotterminal H and the dimmed hot terminal DH to increase the capacitancewhen the intensity level L_(TRGT) is between the low intensity thresholdL_(LOW) and the high intensity threshold L_(HIGH) (e.g., near the middleof the dimming range). The control circuit may open the controllableswitch 325 to disconnect the capacitor C324 between the hot terminal Hand the dimmed hot terminal DH to decrease the capacitance when theintensity level L_(TRGT) is not between the low intensity thresholdL_(LOW) (e.g., 25% intensity) and the high intensity threshold L_(HIGH)(e.g., during the other portions of the dimming range).

FIG. 4 is a simplified block diagram of another example dimmer switch400 (e.g., a digital or “smart” dimmer switch), which may be deployed asthe dimmer switch 100 of FIG. 1 and/or the dimmer switch 200 of FIG. 2.The dimmer switch 400 may comprise a bidirectional semiconductor switch,e.g., a thyristor, such as, a triac 410, which may be coupled between ahot terminal H and a dimmed hot terminal DH. The hot terminal H mayreceive a hot voltage V_(H) from an AC power source (e.g., the AC powersource 105). The triac 410 may comprise a first main terminalelectrically coupled to the hot terminal H and a second main terminalelectrically coupled to the dimmed-hot terminal DH. The triac 410 maycomprise a gate terminal (e.g., a control input), which may receivecontrol signals for rendering the triac conductive. Alternatively, thebidirectional semiconductor switch of the dimmer switch 400 may comprisea field-effect transistor (FET) in a rectifier bridge, two FETselectrically coupled in anti-series connection, and/or one or moreinsulated gate bipolar junction transistors (IGBTs).

The dimmer switch 400 may comprise a mechanical air-gap switch 412electrically coupled to the hot terminal H and in series with the triac410, such that the electrical load may be turned off when the switch isopen. The air-gap switch 412 may be mechanically coupled to an actuatorof a user interface of the dimmer switch 400, such that the switch maybe opened and closed in response to actuations of the actuator.

When the air-gap switch 412 is closed, the triac 410 may be controlledto generate a phase-control voltage V_(PC) (e.g., a forwardphase-control voltage) and conduct a load current I_(LOAD) through anelectrical load (e.g., the lighting load 101 shown in FIG. 1) forcontrolling of the amount of power delivered to the electrical load. Thetriac 410 may become non-conductive when the magnitude of the loadcurrent I_(LOAD) conducted through the triac drops below a rated holdingcurrent of the triac. The phase-control voltage V_(PC) may have amagnitude approximately equal to zero volts at the beginning of eachhalf cycle during a non-conduction time T_(NC), and may have a magnitudeapproximately equal to the magnitude of the AC line voltage V_(AC) ofthe AC power source 105 during the rest of the half cycle, e.g., duringa conduction time T_(CON).

The dimmer switch 400 may comprise a control circuit 414, e.g., adigital control circuit having a processor, such as, a microprocessor, aprogrammable logic device (PLD), a microcontroller, an applicationspecific integrated circuit (ASIC), a field-programmable gate array(FPGA), or any suitable controller or processing device. The controlcircuit 414 may be responsive to actuators 416 (e.g., the rocker switch116 and/or the intensity adjustment actuator 118). The dimmer switch 400may comprise a memory (not shown) configured to store operationalcharacteristics of the dimmer switch (e.g., a low-end intensity L_(LE),a high-end intensity L_(HE), etc.). The memory may be implemented as anexternal integrated circuit (IC) or as an internal circuit of thecontrol circuit 414.

The processor of the control circuit 414 may enable the dimmer switch400 to offer advanced features and functionality to a user. For example,the user may be able adjust the features and functionality of the dimmerswitch 400 using an advanced programming mode. The control circuit 414may be configured to enter the advanced programming mode in response toone or more actuations of the actuators 416. For example, the user mayadjust the low-end intensity L_(HE) and the high-end intensity L_(HE)between which the control circuit 320 may control the target intensityL_(TRGT) of the LED light source 104. A dimmer switch having an advancedprogramming mode is described in greater detail in commonly-assignedU.S. Pat. No. 7,190,125, issued Mar. 13, 2007, entitled PROGRAMMABLEWALLBOX DIMMER, the entire disclosure of which is hereby incorporated byreference. In addition, the operation of the dimmer switch 400 may beconfigured using an external programming device (such as a smart phone,a tablet, or a laptop) as described in greater detail incommonly-assigned U.S. Pat. No. 9,544,977, issued Jan. 10, 2017,entitled METHOD OF PROGRAMMING A LOAD CONTROL DEVICE USING A SMARTPHONE, the entire disclosure of which is hereby incorporated byreference.

The dimmer switch 400 may comprise a power supply 430 configured toconduct a charging current I_(CHRG) through the electrical load (e.g.,the LED driver 102) for generating a first DC supply voltage V_(CC1)(e.g., approximately 8 volts) and a second DC supply voltage V_(CC2)(e.g., approximately 4 volts) for powering the control circuit 414. Bothof the first and second DC supply voltages V_(CC1), V_(CC2) may bereferenced to a circuit common and the power supply 430 may conduct thecharging current I_(CHRG) through circuit common. For example, the powersupply 430 may comprise a resistor-zener power supply for generating thefirst DC supply voltage V_(CC1) and a high-efficiency switching powersupply for generating the second DC supply voltage V_(CC2).Alternatively, the power supply 430 may comprise one or more linearregulators, or other suitable power supply, in addition to anycombination of linear regulators, switching power supplies, andresistor-zener power supplies. As shown in FIG. 4, the dimmer switch 400may not comprise a neutral terminal (e.g., to be coupled to the neutralside N of the AC power source 105) thus requiring that the power supply430 conducts the charging current I_(CHRG) through the electrical load.The power supply 430 also does not conduct any portion of the chargingcurrent I_(CHRG) through an earth ground connection as shown in FIG. 4.

The dimmer switch 400 may also comprise a zero-cross detect circuit 432that may generate a zero-cross signal V_(ZC) that indicates thezero-crossings of the AC line voltage. Since the dimmer switch 400 maynot comprise a neutral connection and/or an earth ground connection, thezero-cross detect circuit 432 may be coupled between the hot terminal Hand the dimmed-hot terminal DH and may be responsive to a dimmer voltageV_(DIM) (e.g., the voltage across the dimmer switch 400). The zero-crossdetect circuit 432 may be configured to drive the zero-cross signalV_(ZC) low towards circuit common when the magnitude of the dimmervoltage V_(DIM) rises above a zero-cross threshold (e.g., approximately30 volts) during the positive half-cycles of the AC power source. Thecontrol circuit 414 may receive the zero-cross signal V_(ZC) and maydetermine when to render the triac 410 conductive each half cycle basedon the indications of the zero-crossings from the zero-cross signal. Thecontrol circuit 414 may sample the zero-cross signal V_(ZC) during azero-cross window once every line cycle (or every half cycle) to lookfor an indication of a zero-crossing. For example, a falling edge of thezero-cross signal V_(ZC) at the beginning of the positive half-cyclesmay indicate a zero-crossing of the AC power source. The control circuit414 may determine when to sample the zero-cross signal V_(ZC) during azero-cross window based on a previous zero-crossing time (e.g.,approximately the period of one line cycle from the previouszero-crossing time). If the control circuit 414 does not detect anindication of a zero-crossing in a predetermined number of sequentialline cycles (e.g., approximately three line cycles), the control circuitmay reset.

The dimmer switch 400 may also comprise a neutral terminal (not shown)adapted to be coupled to a neutral connection (e.g., the neutral side ofthe AC power source). For example, the power supply 430 may be coupledbetween the hot terminal H and the neutral terminal, such that the powersupply may not conduct the charging current I_(CHRG) through theelectrical load. In addition, the dimmer switch 400 may comprise aneutral terminal zero-cross detect circuit (not shown) that may becoupled between the hot terminal H and the neutral terminal forgenerating a zero-cross signal indicating the zero-crossings of the ACpower source.

If the dimmer switch 400 comprises a neutral terminal, the dimmer switch400 may comprise both the zero-cross detect circuit 432 coupled betweenthe hot terminal and the dimmed hot terminal and the neutral terminalzero-cross detect circuit coupled between the hot terminal H and theneutral terminal. The dimmer switch 400 may be configured to determineif the neutral terminal is electrically connected to the neutral side ofthe AC power source in response to the neutral terminal zero-crossdetect circuit. The dimmer switch 400 may be configured to operate in atwo-wire mode in which the control circuit 414 is responsive to thezero-cross circuit 432 coupled between the hot terminal H and the dimmedhot terminal DH, and in a three-wire mode in which the control circuitis responsive to the neutral terminal zero-cross detect circuit (e.g.,in response to determine that the neutral terminal is connected to theneutral side of the AC power source). An example of a dimmer switchconfigured to operate in two-wire and three-wire modes of operation isdescribed in greater detail in commonly-assigned U.S. Pat. No.7,859,815, issued Dec. 28, 2010, entitled ELECTRONIC CONTROL SYSTEMS ANDMETHODS, the entire disclosure of which is hereby incorporated byreference.

The dimmer switch 400 may also comprise an earth ground terminal (notshown) adapted to be coupled to an earth ground connection. For example,the power supply 430 may be coupled between the hot terminal H and theearth ground terminal for leaking at least a portion of the chargingcurrent I_(CHRG) through the earth ground connection (e.g., the powersupply may not conduct any of the charging current I_(CHRG) through theelectrical load). In addition, the dimmer switch 400 may comprise anearth ground terminal zero-cross detect circuit (not shown) that may becoupled between the hot terminal H and the earth ground terminal forgenerating a zero-cross signal indicating the zero-crossings of the ACpower source.

The dimmer switch 400 may comprise a gate coupling circuit 440 and acontrollable switching circuit 450 electrically coupled in seriesbetween the control circuit 414 and the gate terminal of the triac 410.The gate coupling circuit 440 and the controllable switching circuit 450may operate as a gate current path for conducting pulses of gate currentI_(G) through the gate terminal of the triac 410 to render the triacconductive.

The gate coupling circuit 440 may comprise a voltage-controlledcontrollably conductive device, such as two MOS-gated transistors (e.g.,FETs Q442A, Q442B) coupled in anti-series connection between the gateand a first one of the main terminals of the triac 410 (e.g., the hotterminal H of the dimmer switch). The FETs Q442A, Q442B may compriseMOSFETs or may alternatively be replaced by any suitablevoltage-controlled semiconductor switches, such as, for example, IGBTs.The sources of the FETs Q442A, Q442B may be coupled together through twosource resistors R448, R449 (e.g., each having a resistance ofapproximately 10 Ω). The source resistors R448, R449 may operate tolimit the magnitude of the gate current I_(G) conducted through the gateof the triac 410 to a maximum gate current (e.g., approximately 0.6amp). The junction of the source resistors R448, R449 may provide thecircuit common for the power supply 430 to allow the power supply toconduct the charging current I_(CHRG) through the electrical load.

The gate coupling circuit 440 may comprise first and second gate drivecircuits 444, 446 that allow for independent control the FETs Q442A,Q442B. The control circuit 320 may generate two drive signals V_(DR1),V_(DR2) that are received by the respective gate drive circuits 340, 350for rendering the respective FETs Q442A, Q442B conductive andnon-conductive, such that the triac 410 may be rendered conductive toconduct the load current I_(LOAD) to the electrical load. For example,the control circuit 414 may drive the respective drive signals V_(DR1),V_(DR2) high towards the second supply voltage V_(CC2) to render therespective gate drive circuits 444, 446 conductive. The dimmer switch400 may further comprise a full-wave rectifier bridge that may includethe body diodes of the FETs Q442A, Q442B and diodes D434A, D434B, andmay generate the rectified voltage V_(RECT) that is received by thecontrol circuit 414 and the power supply 430.

The control circuit 414 may generate a switch control signal V_(SW) forrendering the controllable switching circuit 450 conductive andnon-conductive. When the controllable switching circuit 450 isconductive, the control circuit 414 may render the FETs Q442A, Q442Bconductive to allow the gate coupling circuit 440 to conduct a pulse ofgate current I_(G) through the gate terminal of the triac 410 to renderthe triac conductive, e.g., at the firing time each half cycle asdetermined with respect to the previous zero-crossing of the AC linevoltage. When operating in the pulse gate drive mode, the controlcircuit 414 may control the drive signals V_(DR1), V_(DR2) to renderboth of the FETs Q332A, Q332B non-conductive (e.g., after the shortpulse time period T_(PULSE)).

The dimmer switch 400 may comprise a resistor R436, which may have aresistance of, for example, approximately 90.9 Ω and may be coupledbetween the gate and a second one of the main terminals of the triac 410(e.g., to the dimmed hot terminal DH of the dimmer switch). The gatecoupling circuit 440 and the resistor R338 may operate as an alternatepath for conducting the load current I_(LOAD). When operating in theconstant gate drive mode, the control circuit 414 may be configured tocontrol the FETs Q442A, Q442B of the gate coupling circuit 440 toconduct the load current I_(LOAD) to the electrical load after the triac410 becomes non-conductive and before the end of the present half cycle.The control circuit 414 may be configured to render the controllableswitching circuit 450 non-conductive to disconnect the gate terminal ofthe triac 410 from the FETs Q442A, Q442B of the gate coupling circuit440 before the end of each half cycle of the AC line voltage, such thatthe triac is able to commutate off before the end of the half cycle.

The dimmer switch 400 may further comprise a controllable RFI filtercircuit 420. The controllable RFI filter circuit 420 may comprise aninductor L422 (e.g., a filter inductor or choke) coupled in series withthe second main terminal of the triac 410. The controllable RFI filtercircuit 420 may comprise a first capacitor C424 (e.g., a first filtercapacitor) electrically coupled in series a first controllable switch425. The controllable RFI filter circuit 420 may also comprise a secondcapacitor C426 (e.g., a second filter capacitor), which may be coupledin parallel with the series combination of the first capacitor C324 andthe first controllable switch 425 to form a filter capacitor network.The first capacitor C424, the controllable switch 425, and the secondcapacitor C426 may form a capacitive circuit, e.g., a controllablecapacitive circuit.

The controllable RFI filter circuit 420 may also comprises a resistorR428. The filter capacitor network may be coupled in series with theresistor R428 (e.g., a filter resistor) between the hot terminal H andthe dimmed hot terminal. The controllable RFI filter circuit 420 mayfurther comprise a second controllable switch 429 coupled in parallelwith the resistor R428. The control circuit 414 may be configured torender the second controllable switch 429 conductive to short theresistor R428, such that filter capacitor network is coupled between thehot terminal H and the dimmed hot terminal DH (e.g., to configure thecontrollable RFI filter circuit 420 as an LC filter circuit). Thecontrol circuit 414 may be configured to render the second controllableswitch 429 non-conductive to connect the resistor R428 in series withthe filter capacitor network between the hot terminal H and the dimmedhot terminal DH (e.g., to configure the controllable RFI filter circuit420 as an RLC filter circuit). The resistor R428 and the controllableswitch 429 may form a resistive circuit, e.g., a controllable resistivecircuit. In some examples, the resistor R428 and the second controllableswitch 429 may also be omitted from the dimmer switch 400, and forexample, may be replaced with a short to provide a controllable LCcircuit (e.g., so that the controllable RFI filter circuit 420 acts as acontrollable LC circuit). The controllable switches 425 and/or 429 maybe a single transistor (e.g., a FET), an optocoupler, a relay, oranother type of controllable switching circuit.

The control circuit 414 may be configured to render the firstcontrollable switch 425 non-conductive to control the capacitancebetween the hot terminal H and the dimmed hot terminal DH to a firstvalue (e.g., by only coupling the second capacitor C426 between the hotterminal H and the dimmed hot terminal DH), for example, when the secondcontrollable switch 429 is conductive. The control circuit 414 may beconfigured to render the first controllable switch 425 conductive tocontrol (e.g., increase) the capacitance between the hot terminal H andthe dimmed hot terminal DH to a second value (e.g., by coupling thefirst and second capacitors C424, C426 in parallel between the hotterminal H and the dimmed hot terminal DH). In some examples, thecontrollable RFI filter circuit 420 may include the first capacitor C424but not the second capacitor C426. In such examples, the control circuit414 may be configured to render the first controllable switch 425non-conducive and conductive to change the capacitance between the hotterminal H and the dimmed hot terminal DH from a first capacitance(e.g., zero capacitance) to a second capacitance (e.g., the capacitanceprovided by the first capacitor C424), respectively.

The control circuit 414 may be configured to adjust the filteringcharacteristics of the controllable RFI filter circuit 420 (e.g.,control the capacitance and/or resistance of the controllable RFI filtercircuit 420). For example, the control circuit 414 may switch thecontrollable RFI filter circuit 420 between an LC filter circuit and anRLC filter circuit based on, for example, on or more factors, such asthe state of the electrical load (e.g., whether the electrical load ison or off, the intensity of the electrical load, during or after aturn-on period, etc.). Further, as discussed in more detail below, thecontrol circuit 414 may be configured to adjust the filteringcharacteristics of the controllable RFI filter circuit 420 when in anadvanced programming mode.

The control circuit 414 may be configured to control the firstcontrollable switch 425 and/or the second controllable switch 429 of thecontrollable RFI filter circuit 420 in response to the state of theelectrical load (e.g., the state of the triac 410). For example, thecontrol circuit 414 may be configured to render the first controllableswitch 425 non-conductive to couple just the second capacitor C426between the hot terminal H and the dimmed hot terminal DH when theelectrical load is off. Alternatively or additionally, the controlcircuit 414 may be configured to render the second controllable switch429 non-conductive to connect the resistor R428 in series with thefilter capacitor network between the hot terminal H and the dimmed hotterminal DH when the electrical load is off. The control circuit 414 maybe configured to render the first controllable switch 425 conductive tocouple the first and second capacitors C424, C426 in parallel betweenthe hot terminal H and the dimmed hot terminal DH when the electricalload is on. And alternatively or additionally, the control circuit 414may be configured to render the second controllable switch 429conductive to short the resistor R428 between the hot terminal H and thedimmed hot terminal DH when the electrical load is on. In some examples,the control circuit may apply a delay between rendering the firstcontrollable switch 425 conductive and rendering the second controllableswitch 429 non-conductive in response to turning the electrical load on.

The control circuit 414 may be configured to control the firstcontrollable switch 425 and/or the second controllable switch 429 toprovide a different impedance between the hot terminal H and the dimmedhot terminal DH while the dimmer switch 400 is turning on the lightingload (e.g., during a turn-on sequence) than while the dimmer switch 400is in a steady state condition. The control circuit 414 may beconfigured to adjust the impedance (e.g., impedance level) providedbetween the hot terminal H and the dimmed hot terminal DH during aturn-on period after one of the actuators 416 is actuated to turn theelectrical load on and/or after the air-gap switch 412 is closed to turnon the electrical load. For example, the control circuit 414 may beconfigured to render the first controllable switch 425 non-conductive tocouple just the second capacitor C426 between the hot terminal H and thedimmed hot terminal DH during the turn-on period. Alternatively oradditionally, the control circuit 414 may be configured to render thesecond controllable switch 429 non-conductive to connect the resistorR428 in series with the filter capacitor network between the hotterminal H and the dimmed hot terminal DH during the turn-on period. Thecontrol circuit 414 may be configured to render the first controllableswitch 425 conductive to couple the first and second capacitors C424,C426 in parallel between the hot terminal H and the dimmed hot terminalDH after the end of the turn-on period (e.g., during steady state). Andalternatively or additionally, the control circuit 414 may be configuredto render the second controllable switch 429 conductive to short theresistor R428 between the hot terminal H and the dimmed hot terminal DHafter the end of the turn-on period (e.g., during steady state). In someexample, the control circuit may apply a delay between rendering thefirst controllable switch 425 conductive and rendering the secondcontrollable switch 429 non-conductive after the end of the turn-onperiod.

The control circuit 414 may be configured to control the controllableRFI filter circuit 420 to provide a different impedance between the hotterminal H and the dimmed hot terminal DH during different portions ofthe dimming range of the dimmer switch 400. For example, the controlcircuit 414 may close the controllable switch 425 to connect thecapacitor C424 between the hot terminal H and the dimmed hot terminal DHto increase the capacitance when the intensity level L_(TRGT) is betweena low intensity threshold L_(LOW) (e.g., 25% intensity) and a highintensity threshold L_(HIGH) (e.g., 75% intensity) (e.g., near themiddle of the dimming range), such as when the triac 410 is switchingnear the peak of the AC line voltage. Alternatively or additionally, thecontrol circuit 414 may close the controllable switch 429 to decreasethe resistance of the controllable RFI filter circuit 420 when theintensity level L_(TRGT) is between the low intensity threshold L_(LOW)and the high intensity threshold L_(HIGH) (e.g., near the middle of thedimming range). The control circuit may open the controllable switch 425to disconnect the capacitor C424 between the hot terminal H and thedimmed hot terminal DH to decrease the capacitance when the intensitylevel L_(TRGT) is not between the low intensity threshold L_(LOW) andthe high intensity threshold L_(HIGH) (e.g., during the other portionsof the dimming range). Alternatively or additionally, the controlcircuit 414 may open the controllable switch 429 to increase theresistance of the controllable RFI filter circuit 420 when the intensitylevel L_(TRGT) is not between the low intensity threshold L_(LOW) andthe high intensity threshold L_(HIGH) (e.g., during the other portionsof the dimming range). In some example, the control circuit may apply adelay between rendering the first controllable switch 425 conductive andrendering the second controllable switch 429 non-conductive whentransitioning into and out of the middle of the dimming range.

Further, in some examples, the control circuit 414 may be configured tocontrol the controllable RFI filter circuit 420 to provide a differentimpedance between the hot terminal H and the dimmed hot terminal DH atan upper end of the dimming range of the dimmer switch 400 (e.g., whenthe target intensity is above 80%). For example, the control circuit 414may open the controllable switch 425 to disconnect the capacitor C424and/or open the controllable switch 429 to connect the resistor R428between the hot terminal H and the dimmed hot terminal DH when thetarget intensity L_(TRGT) is above an upper threshold of the dimmingrange (e.g., above 80%). And the control circuit 414 may close thecontrollable switch 425 to connect the capacitor C424 and/or close thecontrollable switch 429 to short the resistor R428 between the hotterminal H and the dimmed hot terminal DH when the target intensityL_(TRGT) is below the upper threshold of the dimming range and theelectrical load is on (e.g., between the minimum intensity level and80%). Finally, the control circuit 414 may open the controllable switch425 to disconnect the capacitor C424 and/or open the controllable switch429 to connect the resistor R428 between the hot terminal H and thedimmed hot terminal DH when the electrical load is off.

The dimmer switch 400 may further comprise a dimmer voltage measurementcircuit 460 that may be electrically coupled between the hot terminal Hand the dimmed hot terminal DH (e.g., in parallel with the triac 410 andthe controllable RFI filter circuit 420). The dimmer voltage measurementcircuit 460 may generate a dimmer voltage feedback signal V_(DV) thatmay indicate a magnitude of a voltage developed across the dimmer switch400. For example, the dimmer voltage measurement circuit 460 maycomprise a scaling circuit, such as a resistive divider circuit, suchthat the dimmer voltage feedback signal V_(DV) is a scaled version ofthe voltage developed across the dimmer switch 400.

The control circuit 414 may receive the dimmer voltage feedback signalV_(DV). For example, the control circuit 414 may comprise ananalog-to-digital converter (ADC) for sampling the dimmer voltagefeedback signal V_(DV). The control circuit 414 may be configured toperiodically sample the dimmer voltage feedback signal V_(DV) a numberof times during a sampling window T_(SAMPLE) after the control circuitrenders the triac 410 conductive at the firing time. The control circuit414 may be configured to determine a slope m_(DV) of the dimmer voltagefeedback signal V_(DV) (e.g., a rate of change of the dimmer voltagefeedback signal V_(DV)) while the triac 410 is transitioning from thenon-conductive state to the conductive state. If the magnitude of theslope m_(DV) (e.g., the absolute value of the slope m_(DV)) of thedimmer voltage feedback signal V_(DV) too high (e.g., exceeds apredetermined threshold TH_(SLOPE)), the control circuit 414 may controlthe controllable RFI filter circuit 420 to adjust the filteringcharacteristics of the controllable RFI filter circuit 420. For example,the control circuit 414 may be configured to close the controllableswitch 425 to connect the capacitor C424 between the hot terminal H andthe dimmed hot terminal DH to increase the capacitance of thecontrollable RFI filter circuit 420 when the magnitude of the slopem_(DV) of the dimmer voltage feedback signal V_(DV) exceeds thepredetermined threshold TH_(SLOPE). The control circuit 414 may beconfigured to adjust the filtering characteristics of the controllableRFI filter circuit 420 in response to the slope m_(DV) of the dimmervoltage feedback signal V_(DV) when the dimmer switch 400 is firstpowered on and/or each time that the dimmer switch 400 is powered on(e.g., when the air-gap switch 412 is adjusted from an open state to aclosed state). The control circuit 414 may be configured to store thedetermined filtering characteristics of the controllable RFI filtercircuit 420 (e.g., the states of the controllable switches 425, 429) inthe memory.

Although not illustrated, the controllable RFI filter circuit 420 mayinclude any number of capacitors, resistors, and/or associatedcontrollable switches. Further, in some examples, the controllable RFIfilter circuit 420 may include multiple inductors. For example, thecontrollable RFI filter circuit 420 may include multiple RLC circuitsthat can be controlled by the control circuit 414 to be switched in/outbased on, for example, the state of the electrical load. Further, thecontrollable RFI filter circuit 420 may include more resistors and/orcapacitors and more associated switches that can be controlled by thecontrol circuit 414 to, for example, allow for the control circuit 414to more precisely control the impedance of the controllable RFI filtercircuit 420.

Additionally, it should be appreciated that in some examples, the dimmerswitch 400 may be configured to adjust the operation and/or filteringcharacteristics of the controllable RFI filter circuit 420 using theadvanced programming mode. For example, when the control circuit 414 isplaced in the advanced programming mode, the control circuit 414 mayadjust the impedance (e.g., the capacitance and/or the resistance) ofthe controllable RFI filter circuit 420 in response to a received userinput (e.g., a manual actuation of one or more of the actuators 416). Inaddition, the advanced programming mode may be used to change the loadcontrol device between a plurality of filter modes. For example, thefilter modes may include, but are not limited to a first filter modewhere the second controllable switch 429 is closed and the firstcontrollable switch 425 can be controlled to change capacitance value ofthe controllable RFI filter circuit 420 (e.g., control of thecontrollable switch 429 is disabled), and a second filter mode where thesecond controllable switch 429 can be controlled to change thecontrollable RFI filter circuit 420 between an LC filter circuit and anRLC filter circuit (e.g., control of the controllable switch 429 isenabled). Further, the user may enable or disable the control circuit414 from adjusting (e.g., automatically adjusting) the impedance of thecontrollable RFI filter circuit 420 and/or the filter mode.

FIG. 5 is a flowchart of a filter circuit control procedure 500 that maybe performed by a control circuit of a load control device, such as thecontrol circuit of the dimmer switch 100, the control circuit 214 of thedimmer switch 200, the control circuit 314 of the dimmer switch 300,and/or the control circuit 414 of the dimmer switch 400. The controlcircuit may perform the control procedure 500 periodically. The controlcircuit may determine the state of the electrical load at 510. Forexample, the control circuit may determine the state of the electricalload based on the state of the bidirectional semiconductor switch of theload control device (e.g., based on whether the bidirectionalsemiconductor switch is conductive or non-conductive).

At 520, the control circuit may determine whether the electrical load ison or off. If the control circuit determines that the electrical load isoff at 520, the control circuit may configure a filter circuit (e.g., acontrollable RFI filter circuit, such as the controllable RFI filtercircuit 220, the controllable RFI filter circuit 320, or thecontrollable RFI filter circuit 420) for a first capacitance level at530 and/or a first resistance level at 540 before exiting the controlprocedure 500. If the control circuit determines that the electricalload is on at 520, the control circuit may configure the filter circuit(e.g., the controllable RFI filter circuit) for a second capacitancelevel at 550 and/or a second resistance level at 560 before exiting thecontrol procedure 500. The control circuit may configure the filtercircuit for the second capacitance and/or resistance level by closingone or more controllable switches of the filter circuit (e.g.,controllable switch 325, controllable switch 425, controllable switch429, etc.). In some examples, the second capacitance level may begreater than the first capacitance level, and the second resistancelevel may be less than the first resistance level.

Further, although described with reference to a first and secondcapacitance level and a first and second resistance level, the controlprocedure 500 may be configured with just a first and second capacitancelevel or a first and second resistance level. That is, in some examples,the control procedure 500 may be configured to hold the capacitance orthe resistance constant regardless of whether the control circuitdetermines that the electrical load is on or off (e.g., regardless ofthe state of the electrical load). For instance, the capacitance orresistance may be held constant if, for example, the filter circuit doesnot include a switch and a capacitor/resistor (e.g., if the resistorR428 and the controllable switch 429 are omitted from the controllableRFI filter circuit 420). In addition, the filter circuit may simplycomprise a capacitor coupled in series with a switch (e.g., if thesecond capacitor C426, the resistor R428, and the controllable switch429 are omitted from the controllable RFI filter circuit 420), such thatthe second capacitance level may be essentially no capacitance. Finally,it should be appreciated that in some examples, the control circuit maycontrol the controllable switches simultaneously or with a delay whenconfiguring the filter circuit with the first/second capacitance and/orthe first/second resistance when performing the control procedure 500.

FIG. 6 is a flowchart of a filter circuit control procedure 600 that maybe performed by a control circuit of a load control device, such as thecontrol circuit of the dimmer switch 100, the control circuit 214 of thedimmer switch 200, the control circuit 314 of the dimmer switch 300,and/or the control circuit 414 of the dimmer switch 400. The controlcircuit may perform the control procedure 600 periodically. The controlcircuit may receive an input to turn on the electrical load at 610. Forexample, the control circuit may receive the input to turn on theelectrical load via an actuator of the load control device (e.g., therocker switch 116, the actuator 216, and/or the actuator 416) and/or viaa signal received from a remote switch. At 620, the control circuit maycontrol a bidirectional semiconductor switch (e.g., the triac 410) toturn on the electrical load in response to receiving the input.

In some examples, the control circuit may receive the input to turn onthe electrical load at 610 from actuators that are coupled to thecontrol circuit (e.g., the actuators 416). Moreover, in some examples,the control circuit may receive the input to turn on the electrical loadat 610 from the closing of an air-gap switch, which provides power tothe control circuit (e.g., so the power to the control circuit is theinput to turn on the electrical load). For example, an actuator of thedimmer switch (e.g., a toggle actuator) may be coupled to an air-gapswitch of the dimmer switch (e.g., the air-gap switch 412), and when theair-gap switch is closed, power may be provided to the control circuit,and the control circuit may control the bidirectional semiconductorswitch to turn on the load at 620. Further, in some examples, theair-gap switch may be closed prior to the control circuit receiving theinput to turn on the electrical load, for example, if the load controldevice is a smart dimmer (e.g., the dimmer switch 400), where theair-gap switch is controlled in the closed position even when the loadcontrol device is not powering the electrical load. In such instances,the control circuit may receive the input to turn on the electrical loadat 610 via an actuator (e.g., by detecting an actuation of theactuator), via a wireless control signal, and/or the like.

The control circuit may configure a filter circuit (e.g., a controllableRFI filter circuit) for a first capacitance level and/or a firstresistance level at 630. The control circuit may then wait apredetermined amount of time at 640 (e.g., through use of a delaycircuit). The predetermined amount of time may be based on a particularnumber of line cycles of the AC power source. After the predeterminedamount of time, the control circuit may configure the filter circuit(e.g., the controllable RFI filter circuit) for a second capacitancelevel and/or a second resistance level at 650 before exiting the controlprocedure 600. The control circuit may configure the filter circuit forthe second capacitance and/or resistance level by closing one or morecontrollable switches of the filter circuit (e.g., the controllableswitch 325, the controllable switch 425, the controllable switch 429,etc.). The second capacitance level and second resistance level may begreater than the first capacitance level and first resistance level,respectively.

Further, although described with reference to a first and secondcapacitance level and a first and second resistance level, the controlprocedure 600 may be configured with just a first and second capacitancelevel or a first and second resistance level. That is, in some examples,the control procedure 600 may be configured to hold the capacitance orthe resistance constant regardless of whether the control circuitdetermines that the electrical load is on or off (e.g., regardless ofthe state of the electrical load). For instance, the capacitance orresistance may be held constant if, for example, the filter circuit doesnot include a switch and a capacitor/resistor (e.g., if the resistorR428 and the controllable switch 429 are omitted from the controllableRFI filter circuit 420). In addition, the filter circuit may simplycomprise a capacitor coupled in series with a switch (e.g., if thesecond capacitor C426, the resistor R428, and the controllable switch429 are omitted from the controllable RFI filter circuit 420), such thatthe second capacitance level may be essentially no capacitance. Finally,it should be appreciated that in some examples, the control circuit maycontrol the controllable switches simultaneously or with a delay whenconfiguring the filter circuit with the first/second capacitance and/orthe first/second resistance when performing the control procedure 600.

FIG. 7 is a flowchart of a filter circuit control procedure 700 that maybe performed by a control circuit of a load control device, such as thecontrol circuit of the dimmer switch 100, the control circuit 214 of thedimmer switch 200, the control circuit 314 of the dimmer switch 300,and/or the control circuit 414 of the dimmer switch 400. The controlcircuit may perform the control procedure 700 periodically. The controlcircuit may determine the intensity level L_(TRGT) of the electricalload at 710. For example, the control circuit may determine theintensity level L_(TRGT) of the electrical load based on informationreceived from an intensity adjustment actuator of the load controldevice.

At 720, the control circuit may determine if the intensity levelL_(TRGT) is between a low intensity threshold L_(LOW) (e.g., 25%intensity) and a high intensity threshold L_(HIGH) (e.g., 75%intensity). If the control circuit determines that the intensity levelL_(TRGT) is not between the low intensity threshold L_(LOW) and the highintensity threshold L_(HIGH) at 720, then the control circuit mayconfigure a filter circuit (e.g., a controllable RFI filter circuit) fora first capacitance level at 730 and/or a first resistance level at 740before exiting the control procedure 700. If the control circuitdetermines that the intensity level L_(TRGT) is between the lowintensity threshold L_(LOW) and the high intensity threshold L_(HIGH) at720, then the control circuit may configure the filter circuit (e.g.,the controllable RFI filter circuit) for a second capacitance level at750 and/or a second resistance level at 760 before exiting the controlprocedure 700. The control circuit may configure the filter circuit forthe second capacitance and/or resistance level by closing one or morecontrollable switches of the filter circuit (e.g., the controllableswitch 325, the controllable switch 425, the controllable switch 429,etc.). The second capacitance level and second resistance level may begreater than the first capacitance level and first resistance level,respectively.

Further, although described with reference to a first and secondcapacitance level and a first and second resistance level, the controlprocedure 700 may be configured with just a first and second capacitancelevel or a first and second resistance level. That is, in some examples,the control procedure 700 may be configured to hold the capacitance orthe resistance constant regardless of whether the control circuitdetermines that the intensity level L_(TRGT) is between the lowintensity threshold L_(LOW) and the high intensity threshold L_(HIGH).The control circuit may control the controllable switches simultaneouslyor with a delay when configuring the filter circuit with thefirst/second capacitance and/or the first/second resistance whenperforming the control procedure 700.

FIG. 8 is a flowchart of a filter circuit control procedure 800 that maybe performed by a control circuit of a load control device, such as thecontrol circuit of the dimmer switch 100, the control circuit 214 of thedimmer switch 200, the control circuit 314 of the dimmer switch 300,and/or the control circuit 414 of the dimmer switch 400. The controlcircuit may perform the control procedure 800, for example, at start-upof the load control device (e.g., during a start-up routine executed bythe control circuit when the load control device is first powered onand/or each time that the load control device is powered on). At 810,the control circuit may detect a zero-crossing of the AC line voltage(e.g., in response to the zero-cross signal V_(ZC) generated by thezero-cross detect circuit 432 of the dimmer switch 400). At 820, thecontrol circuit may render a bidirectional semiconductor switch of thedimmer switch (e.g., the triac 410) conductive at a firing time duringthe present half-cycle of the AC line voltage (e.g., as timed from thezero-crossing detected at 810). For example, the control circuit maydetermine a value for the firing time based on information received froman intensity adjustment actuator of the load control device.

The control circuit may be responsive to a dimmer voltage feedbacksignal V_(DC) (e.g., the dimmer voltage feedback signal V_(DC) generatedby the dimmer voltage measurement circuit 460 shown in FIG. 4), whichmay indicate a magnitude of a voltage developed across the dimmerswitch. At 830, the control circuit may periodically sample the dimmervoltage feedback signal V_(DC) a number of times during a samplingwindow T_(SAMPLE) after the control circuit renders the bidirectionalsemiconductor switch conductive at the firing time. At 840, the controlcircuit may determine a slope m_(DV) of the dimmer voltage feedbacksignal V_(DC) while the bidirectional semiconductor switch istransitioning from the non-conductive state to the conductive state byprocessing the values of the dimmer voltage feedback signal V_(DC)sampled during the sampling window T_(SAMPLE). For example, the controlcircuit may perform a least-squares fit on the values of the dimmervoltage feedback signal V_(DC) sampled during the sampling windowT_(SAMPLE) to determine the slope m_(DV) of the dimmer voltage feedbacksignal V_(DV) at 840. In addition, the control circuit may determine theslope of the dimmer voltage feedback signal V_(DC) at 840 from a minimumvalue V_(MIN) and maximum value V_(MAX) of the values of the dimmervoltage feedback signal V_(DC) sampled during the sampling windowT_(SAMPLE) (e.g., m_(DV)=1(V_(MAX)−V_(MIN))/T_(SAMPLE)1).

At 850, the control circuit may determine if the magnitude of the slopem_(DV) (e.g., the absolute value of the slope m_(DV)) of the dimmervoltage feedback signal V_(DV) is greater than or equal to apredetermined threshold TH_(SLOPE). If the magnitude of the slope m_(DV)of the dimmer voltage feedback signal V_(DV) is greater than or equal tothe predetermined threshold TH_(SLOPE) at 850, the control circuit mayconfigure a filter circuit (e.g., a controllable RFI filter circuit) fora first capacitance level at 860 and/or a first resistance level at 870.If the control circuit determines that the magnitude of the slope m_(DV)of the dimmer voltage feedback signal V_(DV) is less than thepredetermined threshold TH_(SLOPE) at 850, the control circuit mayconfigure the filter circuit (e.g., the controllable RFI filter circuit)for a second capacitance level at 880 and/or a second resistance levelat 890. The control circuit may configure the filter circuit for thesecond capacitance and/or resistance level by closing one or morecontrollable switches of the filter circuit (e.g., the controllableswitch 325, the controllable switch 425, the controllable switch 429,etc.). The first capacitance level and first resistance level may begreater than the second capacitance level and second resistance level,respectively. At 895, the control circuit may store the states of thecontrollable switches of the filter circuit (e.g., the controllableswitches 325, 425, 429) in memory before exiting the control procedure800.

Further, although described with reference to a first and secondcapacitance level and a first and second resistance level, the controlprocedure 800 may be configured with just a first and second capacitancelevel or a first and second resistance level. That is, in some examples,the control procedure 800 may be configured to hold the capacitance orthe resistance constant regardless of whether the control circuitdetermines that the intensity level L_(TRGT) is between the lowintensity threshold L_(LOW) and the high intensity threshold L_(HIGH).For instance, the capacitance or resistance may be held constant if, forexample, the filter circuit does not include a switch and acapacitor/resistor (e.g., if the resistor R428 and the controllableswitch 429 are omitted from the controllable RFI filter circuit 420). Inaddition, the filter circuit may simply comprise a capacitor coupled inseries with a switch (e.g., if the second capacitor C426, the resistorR428, and the controllable switch 429 are omitted from the controllableRFI filter circuit 420), such that the second capacitance level may beessentially no capacitance. Finally, it should be appreciated that insome examples, the control circuit may control the controllable switchessimultaneously or with a delay when configuring the filter circuit withthe first/second capacitance and/or the first/second resistance whenperforming the control procedure 800.

1. A load control device for controlling an amount of power delivered from an alternating-current (AC) power source to an electrical load, the load control device comprising: a first terminal adapted to be coupled to the AC power source; a second terminal adapted to be coupled to the electrical load; a bidirectional semiconductor switch coupled in series between the first terminal and the second terminal, the bidirectional semiconductor switch configured to be controlled to a conductive state and a non-conductive state; a filter circuit coupled between the first terminal and the second terminal; and a control circuit configured to render the bidirectional semiconductor switch conductive and non-conductive to control the amount of power delivered to the electrical load, the control circuit further configured to adjust an impedance of the filter circuit.
 2. The load control device of claim 1, wherein the filter circuit comprises a first capacitor and an inductor.
 3. The load control device of claim 2, wherein the inductor is coupled in series between the bidirectional semiconductor switch and the second terminal, and the filter circuit further comprises a controllable switch, the control circuit configured to render the controllable switch conductive and non-conductive to respectively connect and disconnect the first capacitor from the series connection between the first terminal and the second terminal to adjust the impedance of the filter circuit.
 4. The load control device of claim 3, wherein the filter circuit further comprises a second capacitor in series connection between the first terminal and the second terminal, the first capacitor and the second capacitor coupled in parallel between the first terminal and the second terminal.
 5. The load control device of claim 4, wherein the control circuit is configured to render the controllable switch non-conductive to control a capacitance between the first terminal and the second terminal to a first value by coupling only the second capacitor between the first terminal and the second terminal, and render the controllable switch conductive to increase the capacitance between the first terminal and the second terminal to a second value by coupling the first capacitor and the second capacitor in parallel between the first terminal and the second terminal.
 6. The load control device of claim 4, wherein the control circuit is configured to render the controllable switch non-conductive when the electrical load is in an off state, and render the controllable switch conductive when the electrical load is in an on state.
 7. The load control device of claim 4, wherein the first capacitor is coupled in parallel with the second capacitor and the controllable switch.
 8. The load control device of claim 3, further comprising: an air-gap switch; wherein the control circuit is configured to render the controllable switch conductive at the end of a turn-on period after the air-gap switch is closed to turn on the electrical load.
 9. The load control device of claim 8, wherein the control circuit comprises a delay circuit to render the controllable switch conductive at the end of the turn-on period, and the turn-on period comprises a predetermined number of line cycles of the AC power source.
 10. The load control device of claim 3, wherein the control circuit is configured to control the controllable switch of the controllable filter based on the target intensity of the electrical load.
 11. The load control device of claim 1, wherein the filter circuit comprises a first capacitor, an inductor, and a resistor.
 12. The load control device of claim 11, wherein the inductor is coupled in series between the bidirectional semiconductor switch and the second terminal, and the filter circuit further comprises a first controllable switch and a second controllable switch; wherein the resistor and the first controllable switch are coupled in parallel between the first terminal and the second terminal, and the first capacitor and the second controllable switch are coupled in series between the first terminal and the second terminal; wherein the control circuit is configured to render the first controllable switch conductive and non-conductive to respectively disconnect and connect the resistor from the series connection between the first terminal and the second terminal to adjust the impedance of the filter circuit; and wherein the control circuit is configured to render the second controllable switch conductive and non-conductive to respectively connect and disconnect the first capacitor from the series connection between the first terminal and the second terminal to adjust the impedance of the filter circuit.
 13. The load control device of claim 12, wherein the filter circuit further comprises a second capacitor in series connection between the first terminal and the second terminal, the first capacitor and the second capacitor coupled in parallel between the first terminal and the second terminal.
 14. The load control device of claim 13, wherein the control circuit is configured to render the controllable switch non-conductive to control a capacitance between the first terminal and the second terminal to a first value by coupling only the second capacitor between the first terminal and the second terminal, and render the controllable switch conductive to increase the capacitance between the first terminal and the second terminal to a second value by coupling the first capacitor and the second capacitor in parallel between the first terminal and the second terminal.
 15. The load control device of claim 12, wherein the control circuit is configured to render the first controllable switch conductive and the second controllable switch non-conductive when the electrical load is in an off state, and render the first controllable switch non-conductive and the second controllable switch conductive when the electrical load is in an on state.
 16. The load control device of claim 1, further comprising: a measurement circuit configured to generate a feedback signal indicating a magnitude of a voltage developed across the load control device; wherein the control circuit is configured to measure a slope of the feedback signal when the bidirectional semiconductor switch is transitioning from the non-conductive state to the conductive state, and adjust the impedance of the filter circuit in response to the slope of the feedback signal.
 17. The load control device of claim 16, wherein the control circuit is configured to increase a capacitance of the filter circuit between the first terminal and the second terminal when the slope exceeds a predetermined threshold.
 18. The load control device of claim 16, wherein the control circuit is configured to measure the slope of the feedback signal and subsequently adjust the impedance of the filter circuit in response to the slope of the feedback signal during a start-up routine when the control circuit is powered on.
 19. The load control device of claim 1, wherein the filter circuit is further coupled between the bidirectional semiconductor switch and the second terminal.
 20. The load control device of claim 1, wherein the control circuit is configured to adjust the impedance of the filter circuit based on the state of the bidirectional semiconductor switch. 21.-55. (canceled) 